Accompanying with radio communications' transferring to broadband media, an antenna apparatus mounted in a user terminal apparatus, such as a notebook personal computer or a PCMCIA card, is increasingly required to have advanced functions and exhibit a high performance. Examples of necessary functions include a function of removing multipath fading from an arriving radio signal. The multipath fading is such a phenomenon that sudden decrease in received level occurs when a radio wave transmitted from one signal source is propagated through a plurality of paths, and when received signals are cancelled at a receiving point with the same amplitudes as and opposite phases to those of each other. When the signal is received by an outdoor radio base station, the spread of an angle of the arriving radio wave through multi paths is relatively narrow. In an indoor environment including the user terminal apparatus, multipath radio waves spreading in all directions over 360 degrees arrives thereto.
A patent document and non-patent documents as cited in the present specification as references are as follows:
(1) Japanese patent laid-open publication No. JP-2002-118414-A (referred to as a first patent document hereinafter);
(2) T. Ohira et al., “Electronically Steerable Passive Array Radiator Antennas for Low-Cost Analog Adaptive Beamforming”, 2000 IEEE International Conference on Phased Array System & Technology pp. 101-104, Dana point, Calif., May 21-25, 2000 (referred to as a first non-patent document hereinafter);
(3) Takashi Ohira, “Basic Theory on ESPAR Antenna Equivalent Weight Vector and Its Gradient”, Technical Report of The Institute of Electronics, Information and Communication Engineers in Japan (referred to as an IEICE hereinafter), published by IEICE, AP2001-16, SAT2001-3, pp. 15-20, May 2001 (referred to as a second non-patent document hereinafter);
(4) Naoki Aoyama et al., “A Simple Diversity Receiver for CDEDM based on Mutual Coupling amongst Antenna Elements”, Proceedings of General Conference of IEICE, published by IEICE, B-5-224, p. 695, Mar. 27-30, 2002 (referred to as a third non-patent document hereinafter);
(5) Takashi Ohira et al., “Equivalent Weight Vector and Array Factor Formulation for Espar Antennas”, Technical Report of IEICE, published by IEICE, AP2000-44, p. 7-14, July 2000 (referred to as a fourth non-patent document hereinafter);
(6) Masahiro Murase et al., “Propagation and Antenna Measurements Using Antenna Switching and Random Field Measurements”, IEEE Transactions on Vehicular Technology, Vol. 43, No. 3, pp. 537-541, August 1994 (referred to as a fifth non-patent document hereinafter);
(7) Hiroyuki Arai, “New antenna Engineering”, pp. 151-155, published by Sougou Denshi Shuppansha, First Edition, Apr. 9, 1996 (referred to as a sixth non-patent document hereinafter);
(8) A. J. Rustako et al., “Performance of Feedback and Switch Space Diversity 900 MHz FM Mobile Radio Systems with Rayleigh Fading”, IEEE Transactions on Communications, Vol. COM-21, pp. 1257-1268, November 1973 (referred to as a seventh non-patent document hereinafter);
(9) A. Afrashteh et al., “Performance of a Novel Selection Diversity Technique in an Experimental TDMA System for Digital Portable Radio Communications”, Conference Record of Globecom '88 Hollywood, pp. 810-814, November 1988 (referred to as an eighth non-patent document hereinafter);
(10) Yoshiaki Akaiwa, “Antenna Selection Diversity for Framed Digital Signal Transmission in Mobile Radio Channel”, Proceeding of 39th IEEE Vehicle Technology Conference, pp. 470-473, 1989 (referred to as a ninth non-patent document hereinafter);
(11) J. G. Proakis, “Digital communications”, 3rd Edition, pp. 274-277, McGraw-Hill, New York, 1995 (referred to as a tenth non-patent document hereinafter);
(12) Makoto Taromaru et al., “A Study on the Mapping from Reactance Space to Equivalent Weight Vector Space of ESPAR Antenna”, Technical Report of IEICE, published by IEICE, RCS2002-179, pp. 43-48, October 2002 (referred to as an eleventh non-patent document hereinafter);
(13) Takashi Ohira et al., “Basic Theory on 2-element Espar Antennas from Reactance Diversity Viewpoint”, Technical Report of IEICE, published by IEICE, AP2002-93, pp. 13-18, October 2002 (referred to as a twelfth non-patent document hereinafter).
By the way, in order to mount an antenna apparatus in a portable terminal apparatus, a PC card or the like, such requirements are imposed on the antenna apparatus as small size and light weight, low cost when the antenna apparatus is accepted as a commercially available consumer product, and operation with low power consumption so that the antenna apparatus can be driven by a battery. As the antenna apparatus that satisfies these requirements, an electronically steerable passive array radiator antenna apparatus is proposed in, for example, each of the first patent document and the first, second and fourth non-patent documents.
The electronically steerable passive array radiator antenna apparatus includes an array antenna which includes a radiating element supplied with a radio signal, six parasitic elements which are provided to be distant from the radiating element at a predetermined interval, and which are not supplied with the radio signal, and variable reactance elements connected to the respective parasitic elements, respectively. By changing reactances of the respective variable reactance elements, a directivity characteristic of the array antenna can be changed. Numeric simulation examples are reported when even a two-element electronically steerable passive array radiator antenna apparatus having a small antenna element interval of one-tenth of the wavelength to be used can exhibit space diversity effect (See the third non-patent document).
The antenna apparatus disclosed in each of the first patent document and the third non-patent document implements an adaptive control processing for finely (substantially continuously) changing reactances so as to determine reactances to be set. The present adaptive control processing accompanies a complicated processing algorithm and a controller for executing the algorithm. Further, the controller needs a DA converter for generating a control voltage to generate a reactance signal to be set to a varactor diode. As a result, the configuration of the antenna apparatus is complicated, and a size and a cost of the apparatus increases.
As shown in FIG. 51, the antenna apparatus as disclosed in the third non-patent document is constituted so that two antenna elements A0 and A1 are arranged in parallel at a predetermined interval d, and so that a variable reactance element 12 of a varactor diode is connected to the antenna element A1 of a parasitic element. If the antenna apparatus is so constituted, a reactance X1 of the variable reactance element 12 can be changed by changing a control voltage applied to the variable reactance element 12 as shown in FIG. 52. However, an input impedance Zin at a feeding port of the antenna element A0 of a radiating element is also disadvantageously changed. In other words, the two-element antenna apparatus has such disadvantages that a change in the input impedance of the antenna apparatus is large relative to a change in the connected reactance, and in that an impedance matching design of the antenna apparatus including up to a feeding system is difficult.
A method for controlling the reactance into two states, and for selecting the reactance when a larger received power is obtained in a two-element electronically steerable passive array radiator antenna apparatus is disclosed in, for example, the twelfth non-patent document. However, a control method for a three-element electronically steerable passive array radiator antenna apparatus is complicated and not established yet.
Further, the three-element electronically steerable passive array radiator antenna apparatus is desired to be small in size, light in weight, and thin.
It is a first object of the present invention to provide an electronically steerable passive array radiator antenna apparatus including three or more elements, which is quite simple in hardware for configuration and steering as compared with conventional art, which can greatly improve an antenna gain with a multipath fading, and which can keep an input impedance of an antenna substantially unchanged with a change in a element value of a variable reactance element.
It is a second object of the present invention to provide an electronically steerable passive array radiator antenna apparatus including three elements, which is small in size, light in weight, and thin.